The principles of the silver complex diffusion transfer reversal process, hereinafter called DTR-process, have been described e.g. in U.S. Pat. No. 2,352,014 and in the book "Photographic Silver Halide Diffusion Processes" by Andre Rott and Edith Weyde--The Focal Press--London and New York, (1972).
In the DTR-process non-developed silver halide of an information-wise exposed photographic silver halide emulsion layer material is transformed with a so-called silver halide solvent into soluble silver complex compounds which are allowed to diffuse into an image-receiving element and are reduced therein with a developing agent, generally in the presence of physical development nuclei, to form a silver image having reversed image density values ("DTR-image") with respect to the black silver image obtained in the exposed areas of the photographic material.
A DTR-image bearing material can be used as a planographic printing plate wherein the DTR-silver image areas form the water-repellant ink-receptive areas on a water-receptive ink-repellant background.
The DTR-image can be formed in the image-receiving layer of a sheet or web material which is a separate element with respect to the photographic silver halide emulsion material (a so-called two-sheet DTR-element) or in the image-receiving layer of a so-called single-support-element, also called mono-sheet element, which contains at least one photographic silver halide emulsion layer integral with an image-receiving layer in waterpermeable relationship therewith. It is the latter mono-sheet version which is preferred for the preparation of offset printing plates by the DTR method. For example the United Kingdom Patent Specification 1,241,661 discloses a method for the production of a planographic printing plate consisting of a sheet material comprising an outer hydrophilic colloid layer on the surface of which there is concentrated a silver image stemming from an underlying exposed silver halide emulsion layer by the silver complex diffusion transfer reversal process. The silver image formed on the surface is suitable for printing images in a lithographic printing process using a dampening liquid.
Supports that are commonly used for lithographic printing plates are metal supports such as e.g. aluminum and flexible supports such as e.g. paper or organic resin supports such as e.g. polyester. Metal supports are generally used for high quality printing and print jobs that require a large number of copies, typically around 100000. Lithographic printing plates having a flexible support are generally used for print jobs requiring a medium print quality and only a limited number of copies typically around 10000.
An important problem that occurs with lithographic printing plates having a flexible support is that during startup of the printing process the plate tends to enlarge somewhat until an equilibrium state is reached. As a consequence the first copies will not be of acceptable quality and have to be disposed of. This problem is particular apparent when images having continuous tones in particular color images have to be reproduced.
EP-A 674227 discloses that the number of copies that have to be disposed of at the start of the printing job when using a printing plate obtained from an imaging element according to the DTR-process having a flexible support can be reduced by using as a support of the imaging element a polyester film e.g. a polyethylene therephthalate film having a thickness between 0.1 mm and 0.35 mm and having a Young-modulus (E-modulus) of at least 4300N/mm.sup.2. Although printing plates obtained from said imaging elements give a better result, there is clearly still room for further improvement.